Following production or, later, when in operation, many electrical elements must have their electrical conductivity tested. To this end, resistance measurements can be carried out. Typical examples of such measurements are resistance measurements of switches or isolators for high-voltage networks or medium-voltage networks in the closed state.
A measuring technique for a resistance measurement on such switches, designed for high currents, is micro-ohm measurement. In micro-ohm measurement a direct current with a relatively high current strength, for example with a current strength of between 10 and 1,000 Amperes, is impressed by the test object. By means of separate terminals the drop in voltage is measured over a portion on the test object.
Such measurement techniques, in which separate measurement contacts are used to impress the current and to capture the drop in voltage, are also referred to as four-point measurements. Such methods have the advantage that contact resistances between the measurement contacts and the test object have only a slight effect on the measured resistance. In the contacts used to impress the current, the resistance plays no role because only the necessary driving voltage at the voltage source increases as a result. But this has no effect on the measured voltage. At the contacts via which the voltage is measured, only a low current flows, since the voltage measuring device is high-impedance. Therefore, there is no significant drop in voltage on the voltage terminals either, which could falsify the measured voltage.
The spatial area, for measurement of the voltage dropping across it, is defined by the position of the contacts which are applied to the test object for the voltage measurement. With very many test objects in power engineering, such as, for example, high-voltage switches, the areas to which the contacts for measurement are applied, are solid metal parts with high conductivity. The relevant resistances are for example evident at screw positions, abutting or clamping points or switching contacts of closed switches or isolators. The precise positions at which the contacts are applied, do not significantly influence the measurement result of the resistance measurement. But it is very important to keep the contact made for the current path and the contact made for the voltage path totally separate.
With many test objects a connection point may be present, having an internal thread. In normal operation such a connection point can have a good electrical conductor connected to it. Often, however, the connection point has small dimensions. This makes it difficult to keep the contact made for the current path and the contact made for the voltage path totally separate, if a contact for the current path and a contact for the voltage path are to be applied at the connection point.